The present invention relates to a sensor and related circuitry for detecting movement or inclination of an object relative to either of two mutually perpendicular axes.
More specifically, the present invention relates to a sensor incorporating four parallel plate capacitors arranged to detect changes in capacitance proportional to angular changes of the sensor relative to each of the two mutually perpendicular axes. These changes in capacitance are electrically conditioned to output voltage signals proportional to the angular changes of the sensor.
Inclination sensors utilizing electrostatic capacitive effects are known in the art. For example, U.S. Pat. No. 4,707,927 discloses an inclination and acceleration sensor having two spaced apart hemispherical domes with the space between the domes containing a viscous fluid. One side of the construction supports a common electrode, while the other side supports a plurality of detecting electrodes. In its normal orientation, the sensor is arranged such that the hemispherical domes are upwardly concave so that the viscous fluid collects at the center or lowermost point of the spherical sensor, with a ring of air surrounding the fluid. By means of the movement of the viscous fluid which occurs during inclination of the sensor, electrostatic capacity between the common electrode and each of the detecting electrodes varies, and this variance is detected and output in the form of voltage signals.
U.S. Pat. No. 4,422,243 discloses a dual axis capacitive inclination sensor wherein a lower common electrode is mounted on a flat, lower surface, and a plurality of upper electrodes are mounted on a flat outer surface of the upper part of a transducer. The cavity between the electrodes has a partially spherical surface.
Other capacitive type sensors are disclosed in U.S. Pat. No. 4,377,912; UK Patent Application 2 070 774A; European Patent Application 0 196 735; PCT Publication WO 83/01304; Swedish Patent SE456040; and German Patent DE3406174C2.
The present invention relates to a novel and low cost construction for a two axis capacitive inclination sensor, and to related circuitry which provides reliable and accurate indication of angular movement about either of two mutually perpendicular axes.
In one exemplary embodiment of the invention, an outer housing is provided which has a substantially upwardly spherical portion and a peripheral base or flange portion, preferably constructed of an injection molded, electrically conductive plastic material. An inner housing is provided in the form of a thin film of non-conductive plastic material, which is supported and maintained in a substantially spherical shape by a pair of plastic arches which are integrally molded as reinforcing ribs on the underside of the film, extending perpendicular to one another, intersecting at the apex of a dome formed thereby. The arrangement of the ribs or arches separates the underside of the film into four discrete, substantially identical surface areas or sections which are coated with a conductive material. Each area or section is electrically connected to a circuit board or card through elastomeric connectors. The outer housing and inner housing are hermetically sealed about their respective flange areas, but with the spherical portions uniformly spaced from each other to form a cavity therebetween which is partially filled with a dielectric fluid, preferably to approximately 50% of the available volume within the cavity. In this arrangement, the dielectric fluid is normally concentrated about the lowermost peripheral portion of the cavity leaving a pocket of air or bubble lying above the fluid and substantially in the center of the sensor, at its apex.
As a result of this construction, it will be appreciated that four variable parallel plate capacitors are formed wherein the outer housing forms one electrode plate common to all four capacitors, and wherein the conductive coating on the four concave sections on the underside of the inner housing defines detecting electrodes, with the dielectric fluid 40 located between the common plate and the insulating, outer surface of the inner housing. The manner in which the conductive coating is applied to the underside of the inner housing is not critical to the invention, but it will be appreciated that coating the entire underside, including the reinforcing ribs or arches, and then selectively removing the coating along the underside of the respective ribs or arches is a simple and economical approach.
It will be understood by those skilled in the art that as the sensor tilts about a roll axis or a pitch axis, the amount of coverage of each of the coated sections by the dielectric fluid within the cavity varies, and this variance is detected and output in the form of DC voltage signals proportional to the angular movement of the sensor, as will be described in further detail below.
In a second exemplary embodiment of the invention, an outer housing is provided which is similar to the above described outer housing but which may be formed of a stamped sheet of ferrous or non-ferrous metal, with aluminum being preferred. Otherwise, the construction is similar in that the outer housing includes a spherical portion and a peripheral base or flange portion. The inner housing of this alternative embodiment also has a spherical central portion and a peripheral flange portion, but the inner housing can be thermally formed of a sheet material, or it can be molded into the desired shape such that no reinforcing ribs or arches utilized in the first embodiment are required. In this alternative embodiment, the concave or underside surface of the inner housing is selectively coated (or silk screened, or by other suitable means) with conductive material in a pattern which describes four detecting electrodes symmetrically arranged about the pitch axis and roll axis. The conductive coating on the inner concave surface of the inner housing extends into the peripheral flange portion to facilitate electrical contact with an associated circuit board or card.
Another feature of this alternative embodiment is that the outer housing is provided with a fill hole to facilitate introduction of dielectric fluid into the cavity between the inner and outer housings, which fill hole may be sealed with a plug. Alternatively, fluid may be added before the inner and outer housings are sealed.
Another feature of this alternative embodiment is the provision of a force distribution ring which, in conjunction with a plastic cover located beneath the sensor, provides attachment surfaces for fasteners which effect a clamping action between the distribution ring, the lower plastic cover and the peripheral flanges of the inner and outer housings.
Otherwise, the operation of this alternative embodiment is substantially the same as the first described embodiment.
Thus, it may be seen that the invention comprises in its broader aspects a sensor comprising a spherically shaped outer member having an outer convex surface and an inner concave surface defining a common electrode, a correspondingly shaped inner member having an outer convex insulating surface and an inner concave electrically conductive surface, and located within the outer member substantially uniformly spaced therefrom to form a cavity therebetween, a dielectric fluid contained in the cavity; the inner concave electrically conductive surface being divided into a plurality of detecting electrodes.
In addition to the sensor construction per se, the invention also relates to the provision of a suitable electronic circuit for utilization with the above described sensors.
The electronic circuit of the preferred embodiment is characterized by the utilization of a single oscillator for each axis of the sensor, and automatic switching between two capacitors after each single complete cycle of the oscillator, as described further herein.
Specifically, in the preferred embodiment sensor in accordance with the present invention two capacitors change capacitance for a given change in position along one of the sensing axes (e.g., either the pitch or the roll axis). An oscillator is operatively connected to one of the two capacitors and produces one period of an oscillation output signal--wherein the duration of the period is responsive to the capacitance of the one capacitor. At the conclusion of the oscillation period, the one capacitor is operatively disconnected from the oscillator and the other capacitor is operatively connected in its place. The oscillator then produces a further period of oscillation output signal--wherein the duration of this further period is responsive to the capacitance of this other capacitor.
An output circuit (a flip flop, a low pass filter, and an output buffer) may convert the oscillator output signal to a voltage level responsive to the relative durations of the two oscillator periods--and thus to the capacitances of the two sensor capacitors.
Other objects and advantages of the invention will be apparent from the detailed description of the invention which follows.